One molecule of CO2 from the hydrogencarbonate ions of seawater is released, together with a molecule of water, during the calcification (biomineralisation) reaction. Seawater is over-saturated with calcium ions and its concentration of hydrogencarbonate largely dominates that of carbonate and dissolved free CO2. In these conditions, the molecule of CO2 on the righthand side of the above scheme, if it is released to seawater during the biomineralisation of shells (which is very doubtful, as explained below), will react with water, forming carbonic acid which will dissociate forming hydrogencarbonate and hydrogen ions (protons) that would be available for marine calcifiers to form more CaCO3. Alternatively, the carbonic acid can dissociate to form a carbonate ion and two hydrogen ions. These electrolyte dissociations and associations are described by the following schemes: This release of hydrogen ions is usually interpreted as causing potentially damaging seawater acidification. Emerson and Hedges describe carbonate dynamics (their Chapter 4, Carbonate Chemistry) in terms that can be paraphrased: “As organisms form their shells from Ca and carbonate, alkalinity is being removed. This causes the pH to drop which alters the speciation of the inorganic carbonate system to alter in favour of CO2. Thus, the CO2 concentration increases and any gradient driving the gas to the atmosphere increases.” Which is not so different from the following wording in Gattuso et al.
Author(s) Details:
David Moore
Department of Biology, Medicine and Health, School of Biological Sciences, The University of
Manchester, UK.
Matthias Heilweck
Independent Researcher, F-68240, Kaysersberg, France.
William Burton Fears
Department of Medicine, Southwestern Medical School, Dallas, Texas and Founding Fellow of the
American College of Endocrinology, USA.
Peter Petros
Kaapa Biotech Oy, Teilinummentie 4, 09120 Karjalohja, Finland.
Samuel J Squires
Department of Biology, Medicine and Health, The University of Manchester, UK.
Elena Tamburini
Department of Environmental and Prevention Sciences, University of Ferrara, Ferrara, Italy
Robert Paul Waldron
Independent Researcher, LA-70448, Mandeville, Louisiana, USA.
Recent Global Research Developments in Ocean Acidification Impact on Calcifiers
- Vulnerability to OA: The SO is likely to be one of the first and most severely affected regions by OA. Since the industrial revolution, approximately 30% of anthropogenic CO₂ has been absorbed by the global oceans. Average surface seawater pH levels have already decreased by 0.1 and are projected to decline by around 0.3 by the year 2100. OA shallows the saturation horizon, which is the depth below which calcium carbonate (CaCO₃) dissolves. This process increases the vulnerability of many resident marine calcifiers to dissolution [1].
- Mineralogical Composition Matters: The negative impact of OA may be seen first in species depositing more soluble CaCO₃ mineral phases, such as aragonite and high-Mg calcite (HMC). Ocean warming could further exacerbate the effects of OA in these particular species. Taxa with calcitic, aragonitic, and HMC skeletons could be at greater risk, while low-Mg calcite (LMC) species may be more resilient to these changes [1].
- Calcification Rates: Recent research suggests that the reduction in seawater pH mainly affects calcification rates (the production and deposition of CaCO₃), rather than the reduction in [CO₃²⁻]. Predictions indicate a drop in calcification rates by 30% to 40% by mid-century due to OA [2].
- Species Sensitivity: While many calcifiers (e.g., echinoderms, crustaceans, and cephalopods) appear tolerant to near-future ocean acidification (pH ≈ 7.8 by 2100), coccolithophores, calcifying algae, and corals seem to be more sensitive [3].
References
- Figuerola B, Hancock AM, Bax N, Cummings VJ, Downey R, Griffiths HJ, Smith J and Stark JS (2021) A Review and Meta-Analysis of Potential Impacts of Ocean Acidification on Marine Calcifiers From the Southern Ocean. Front. Mar. Sci. 8:584445. doi: 10.3389/fmars.2021.584445
- Figuerola, B., Hancock, A. M., Bax, N., Cummings, V., Downey, R., Griffiths, H. J., … & Stark, J. S. (2020). Predicting potential impacts of ocean acidification on marine calcifiers from the Southern Ocean. bioRxiv, 2020-11.
- Leung, J. Y., Zhang, S., & Connell, S. D. (2022). Is ocean acidification really a threat to marine calcifiers? A systematic review and meta‐analysis of 980+ studies spanning two decades. Small, 18(35), 2107407.